A motor failure determination device has been disclosed in PTL 1 that can determine that a failure has occurred in an electric motor while performing motor drive that supplies electricity to the electric motor from a direct current power supply based on on and off operations of a switching element. The motor failure determination device monitors the electric potential at a contact point between the electric motor and the switching element and determines that the electric motor has failed based on a back electromotive force that arises in the motor when the supply of electricity to the electric motor is switched from on to off. Specifically, in a case where the shaft of the electric motor is locked, the electric motor does not turn, so the value of the back electromotive force that arises when the supply of electricity to the electric motor is switched from on to off is different from the value when the motor is in a normal state. The locking of the electric motor shaft is detected based on the value of the back electromotive force.